Usage of Hazelcast in WSO2 Identity Server

By definition; Caching is the process to store data in a temporary location for a specific period of time. It improves the performance of any type of application.

Like most of the applications out there, WSO2 IS also uses database(s) to store information such as user data, configuration parameters, session information, etc. But reading data directly from the DB all the time reduces application performance due to network latency between DB and application servers, and also the databases tend to perform slower with the growth of data.

Caching comes to save application developers from this situation in order to reduce the number of interactions with DB. Caching has certain advantages and disadvantages that you need to evaluate when deciding on your caching strategy.

Advantages

Disadvantages

Applications are scaled horizontally when good performance and reliability are critical. But here comes the problem with caching; the cache coherence problem.

For example, let’s assume there are two WSO2 IS nodes deployed and node A issues an access token and then a token revocation request comes to node B. Still, node A is not aware of this cache revocation event and it still keeps track of the previously issued access token as a valid one in its cache.

Now the old access token will still be valid according to node A even though it is already revoked by node B until the access token cache object in node A expires.

But later on, it was decided to avoid using distributed shared memory, but keep caches local to each node in the cluster and use messages to cluster members in order to notify about cache invalidations when required. This was due to many practical issues that are related to configuring and running distributed caching properly where the network is not tightly controlled, distributed caching fails in unexpected ways.

There are multiple clustered deployment cache scenarios and the recommended approach is “All caches are local with distributed cache invalidation”.

Local Caching: Enabled
Distributed Caching: N/A
Hazelcast Clustering: Enabled
Distributed Invalidation: Enabled

With the above approach, WSO2 IS employs Hazelcast as the primary method of implementing cluster messages while using distributed caching in a simple setup. Cache invalidation uses Hazelcast messaging to distribute the invalidation message over the cluster and invalidate the caches properly.

If there are multiple WSO2 IS nodes in the deployment, the cache coherence problem is inevitable. One could think of disabling all the cache layers. But it is a huge sacrifice of performance.

Additionally, this blog provides a step-by-step on setting up a cluster with the WKA membership scheme on a local machine.

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